The present invention generally relates to the preparation of semiconductor grade single crystal silicon which is used in the manufacture of electronic components. More particularly, the present invention relates to a process for producing a single crystal silicon ingot in which at least a segment of the constant diameter portion of the ingot is substantially devoid of agglomerated intrinsic point defects, wherein v/G.sub.0 is allowed to vary over the length of the segment as a result of controlling the manner in which the segment cools to a temperature at which agglomerated intrinsic defects would otherwise form.
Single crystal silicon, which is the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the so-called Czochralski ("Cz") method. In this method, polycrystalline silicon ("polysilicon") is charged to a crucible and melted, a seed crystal is brought into contact with the molten silicon and a single crystal is grown by slow extraction. After formation of a neck is complete, the diameter of the crystal is enlarged by decreasing the pulling rate and/or the melt temperature until the desired or target diameter is reached. The cylindrical main body of the crystal which has an approximately constant diameter is then grown by controlling the pull rate and the melt temperature while compensating for the decreasing melt level. Near the end of the growth process but before the crucible is emptied of molten silicon, the crystal diameter must be reduced gradually to form an end-cone. Typically, the end-cone is formed by increasing the crystal pull rate and heat supplied to the crucible. When the diameter becomes small enough, the crystal is then separated from the melt.
In recent years, it has been recognized that a number of defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Such defects arise, in part, due to the presence of an excess (i.e., a concentration above the solubility limit) of intrinsic point defects in the crystal lattice, which are vacancies and self-interstitials. Silicon crystals grown from a melt are typically grown with an excess of one or the other type of intrinsic point defect, either crystal lattice vacancies ("V") or silicon self-interstitials ("I").
Vacancy-type defects are recognized to be the origin of such observable crystal defects as D-defects, Flow Pattern Defects (FPDs), Gate Oxide Integrity (GOI) Defects, Crystal Originated Particle (COP) Defects, crystal originated Light Point Defects (LPDs), as well as certain classes of bulk defects observed by infrared light scattering techniques such as Scanning Infrared Microscopy and Laser Scanning Tomography. Also present in regions of excess vacancies are defects which act as the nuclei for ring oxidation induced stacking faults (OISF). It is speculated that this particular defect is a high temperature nucleated oxygen agglomerate catalyzed by the presence of excess vacancies.
Defects relating to self-interstitials are less well studied. They are generally regarded as being low densities of interstitial-type dislocation loops or networks. Such defects are not responsible for gate oxide integrity failures, an important wafer performance criterion, but they are widely recognized to be the cause of other types of device failures usually associated with current leakage problems.
It is believed that the type and initial concentration of these point defects in the silicon are determined as the ingot cools from the temperature of solidification (i.e., about 1410.degree. C.) to a temperature greater than about 1300.degree. C. That is, the type and initial concentration of these defects are controlled by the ratio v/G.sub.0, where v is the growth velocity and G.sub.0 is the average axial temperature gradient over this temperature range. Referring to FIG. 1, for increasing values of v/G.sub.0, a transition from decreasingly self-interstitial dominated growth to increasingly vacancy dominated growth occurs near a critical value of v/G.sub.0 which, based upon currently available information, appears to be about 2.1.times.10.sup.-5 cm.sup.2 /sK, where G.sub.0 is determined under conditions in which the axial temperature gradient is constant within the temperature range defined above. At this critical value, the concentrations of these intrinsic point defects are at equilibrium. However, as the value of v/G.sub.0 exceeds the critical value, the concentration of vacancies increases. Likewise, as the value of v/G.sub.0 falls below the critical value, the concentration of self-interstitials increases. If the concentration of vacancies or self-interstitials reaches a level of critical supersaturation in the system, and if the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, will likely occur. Under conventional Czochralski-type growth conditions, the density of vacancy and self-interstitial agglomerated defects is typically within the range of about 1.times.10.sup.3 /cm.sup.3 to about 1.times.10.sup.7 /cm.sup.3. While these values are relatively low, agglomerated intrinsic point defects are of rapidly increasing importance to device manufacturers and, in fact, are now seen as yield-limiting factors in the fabrication of complex and highly integrated circuits.
Preventing the formation of agglomerated intrinsic point defects may be achieved by controlling the growth velocity, v, and the average axial temperature gradient, G.sub.0, such that the ratio of v/G.sub.0 is maintained within a very narrow range of values near the critical value of v/G.sub.0 (see, e.g., FIG. 1, generally represented by range X), thus ensuring that the initial concentration of self-interstitials or vacancies does not exceed some critical concentration at which an agglomeration reaction occurs. However, if control of v/G.sub.0 alone is to be relied upon in order to prevent the formation of agglomerated intrinsic point defects, stringent process control and crystal puller design requirements must be met in order to maintain v/G.sub.0 within this narrow range.
Maintaining v/G within a narrow range of values is not the most commercially practical approach to preventing the formation of agglomerated intrinsic point defects, for a number of reasons. For example, the pull rate is often varied during the growth process in order to maintain a constant diameter of the ingot. Variations in the pull rate, however, result in changes in v which, in turn, impacts v/G.sub.0, causing it to vary axially over the length of the ingot. Similarly, changes in G.sub.0 may occur as well, due to changes in other process parameters. Furthermore, it should be noted that G.sub.0 often changes over time as a result of aging of the components of the hot zone or as a result of the inside of the hot zone becoming coated with, for example, silicon dioxide.
The changes in v and G.sub.0 likewise cause changes in the "target" range for v/G.sub.0 (i.e., the range which limits the initial concentration of intrinsic point defects such that agglomerations do not occur), unless a corresponding and offsetting change is made in G.sub.0 or v, respectively. Therefore, if a given crystal puller is to be utilized to grow a series of ingots, the temperature profile of that crystal puller must be continuously monitored and the process conditions repeatedly modified as changes in v or G.sub.0 dictate. Such an approach is both time consuming and costly.